1. Field of the Invention
The present invention relates to a multi-layer circuit board; more particularly to a multi-layer circuit board having a plurality of ground shielding walls.
2. Description of Related Art
Due to the market demand for the size of digital electronic products being smaller and the speed faster, the design of high speed signal circuit board has been increasingly emphasized. Please refer to FIG. 1 which illustrates a four-layer circuit board as an example. The first layer of the multi-layer circuit board 10 is used as a signal layer 12 for mounting device; the second layer is a ground layer 14; the third layer is a power layer 16; and the fourth layer is a signal layer for welding 18. The first, second, third and fourth layer are covered with dielectric layers 20 respectively to separate each layer. In comparison with the power layer 14, the ground layer 12 has better noise-shielding effect. Therefore, all the high speed wirings must be disposed on the signal layer 12 for mounting device.
However, due to the increase of operation frequency and processing speed of electronic products, the above structure for the high-speed signal circuit board can no longer meet the need. In designing a higher-speed signal circuit board, it is getting more important to control noise interference. Please refer to FIGS. 2a and 2b. Another type of high speed signal circuit board 30 prevents the noise interference by using guard traces, such as the ground-shielding-type ground lines 42, to encircle near the high speed signal lines 32. The ground lines 42 further have plated through holes 44 to be connected to the ground layer 34 so as to prevent the poor effect caused by resonance, thereby, preventing the reducing of the signal quality.
For example, please refer to FIG. 3, U.S. Pat. No. 5,372,872, entitled “Multiplayer Printed Circuit Board” which discloses a multi-layer printed circuit board 50 having a through hole 57 passing through the substrate 52 and the pre-preg 54 and electrically connecting to a ground pad 58. The plated through hole 57 is disposed at the intersect point of the plaid-weaved net or within the net. The plaid-weaved net is used as a ground layer of the signal interconnection layer 56 so as to reduce noise interference or cross talk.
However, the above conventional plated through holes are rounded ones. The ground shielding body in the through holes is formed by a plurality of separated cylinders which can not efficiently shield the magnetic field or the inductance of the electric field generated by the high-speed signal lines. Furthermore, the ground shielding cylinders formed by the above conventional plated through holds are likely to generate inductance effect, leading to the poor effect caused by additional resonance.
In addition, the diameter of the conventional plated through hole is usually 100˜200 μm, and the line width is usually 70˜80 μm. To dispose more plated through holes, the guard traces (the ground lines) are often made more elongated. However, if the ground lines are too long the resonance could occur, decreasing the signal transmission quality.
Therefore, to solve the aforesaid problem, there is a need to provide a multi-layer circuit board.